An acceleration sensor was often used to actuate an air bag, and grasped an impact in a collision of an automobile as acceleration. For the automobile, a one-axis (uniaxial) or two-axis (biaxial) function for measuring acceleration in the X-axis direction and/or the Y-axis direction was enough. The acceleration to be measured is so great that an acceleration sensor element for detecting acceleration is also produced sturdily. Recently, the acceleration sensor has found frequent use in portable terminal equipment and robots, and a three-axis (triaxial) acceleration sensor for measuring accelerations in the X-, Y- and Z-axis directions has been in demand in order to detect spatial movements. Also, a high-resolution downsized sensor has been in demand for detection of micro-acceleration.
The acceleration sensor is a method for converting the movement of a flexible beam into an electrical signal, and is roughly classified as the piezoresistance type, the electrostatic capacity type, and the piezoelectric type. Any of these types is selected in consideration of the magnitude of the output of the sensor, the frequency response characteristics, the electromagnetic resistance noise, the linearity of the output, detection of stationary acceleration, and the temperature characteristics. Microfabrication is needed to meet a demand for compactness and high sensitivity. Thus, a photolithography technology is used on a silicon substrate for fabrication, and impurities are implanted into the silicon by a semiconductor technology to form piezoresistance. A semiconductor piezoresistance element type triaxial acceleration sensor produced in this manner has been put to practical use.
In connection with semiconductor piezoresistance element type triaxial acceleration sensors, the applicant filed wide varieties of many patent applicants. Patent Documents 1 to 6 expressly indicate the shapes of weights, the shapes of beams, the arrangement of semiconductor piezoresistance elements, the connection of semiconductor piezoresistance elements, the shapes of junctions between the beams and support frames, and so on. The triaxial acceleration sensor is shown as an exploded perspective view in FIG. 15, a sectional view taken along line XVI A-XVI A in FIG. 15 is shown in FIG. 16A, and an acceleration sensor element used in the triaxial acceleration sensor is shown as a plan view in FIG. 16B. In a triaxial acceleration sensor 200, an acceleration sensor element 100 and a regulating plate 180 are adhered with predetermined spacing within a case 190 with the use of an adhesive 54 such as a resin. Chip terminals 104 of the acceleration sensor element 100 are connected to case terminals 192 by wires 58, and signals from the sensor are taken out from contact or external terminals 194. A case cover 195 is adhered onto the case 190 with the use of an adhesive 55, such as an AuSn solder, for sealing. The triaxial acceleration sensor element 100 is composed of a square support frame 10, a weight 20, and two pairs of beams 30, and the weight 20 is supported in the center of the support frame 10 by the two pairs of beams 30. Semiconductor piezoresistance elements 31x, 31y, 31z are formed in the beams 30. The X-axis semiconductor piezoresistance elements 31x and the Z-axis semiconductor piezoresistance elements 31z are provided in the pair of beams, and the Y-axis semiconductor piezoresistance elements 31y are provided in the other pair of beams. Since the semiconductor piezoresistance elements are arranged at the four ends of the pair of beams and they constitute a bridge circuit, a uniform resistance change in the semiconductor piezoresistance elements due to a temperature change can be cancelled. By changing the mariner of connection of the bridge circuit, moreover, the accelerations in the X-axis, the Y-axis and the z-axis can be separated and detected. In FIG. 16A, a spacing g4 between the lower surface of the weight 20 and the inner bottom surface of the case 190, and a spacing g3 between the upper surface of the weight 20 and the regulating plate 180 regulate the movement of the weight 20 to prevent damage to the beams 30, when excessive acceleration such as impact is imposed on the sensor. The basic structure of the semiconductor piezoresistance element type triaxial acceleration sensor according to the present invention is the same as those present in these patent documents, so that its detailed description is omitted, except where specifically noted. Hereinafter, the semiconductor piezoresistance element type triaxial acceleration sensor and element may be referred to simply as the acceleration sensor or the acceleration sensor element. Moreover, the acceleration sensor and the acceleration sensor element may be used synonymously.
In manufacturing the above acceleration sensor element, it is necessary to process the thickness of the beam highly accurately. Thus, it is common practice to use an SOI (silicon, on insulator) wafer having a thin silicon layer overlaid on the surface of a thick silicon layer via a silicon oxide film layer. Shapes such as beams are processed in the thin silicon layer with the use of the silicon oxide film layer as an etching stopper, and then grooves are processed in the thick silicon layer to separate the support frame and the weight, whereby a structure having the weight supported by the support frame aria the beams comprising the thin silicon layer can be produced.
With the semiconductor piezoresistance element type triaxial acceleration sensor, the weight is provided with notched portions, and the beams are connected to the notched portions, whereby downsizing and high sensitivity can be achieved at the same time. In regard to the acceleration sensor of such a structure, Patent Document 7 to Patent Document 10, for example, offer descriptions. A representative structure thereof is shown as a perspective view in FIG. 17. In an acceleration sensor element 100′, notches 22 are provided in portions of a weight 20′ to which beams 30 are connected, and the beams 30 are connected to the weight 20′ at the farthest ends of the notches 22. Even if the weight 20′ is formed amply inside the support frame 10, therefore, the beams 30 can be lengthened by the lengths of the notches 22, with the result that a sensor with high sensitivity can be obtained, even when its area is small.
Patent Document 1: JP-A-2003-172745
Patent Document 2: JP-A-2003-279592
Patent Document 3: JP-A-2004-184373
Patent Document 4: JP-A-2006-098323
Patent Document 5: JP-A-2006-098321
Patent Document 6: WO2005/062060A1
Patent Document 7: JP-A-11-214705
Patent Document 8: JP-A-2002-296293
Patent Document 9: JP-A-2003-101032
Patent Document 10: JP-A-9-237902
Patent Document 11: JP-A-3-2535
Patent Document 12: JP-A-10-170380
Patent Document 13: JP-A-2000-46862